Amphiphile-based formulations show considerable potential in the delivery of many substances, especially for in vivo delivery to the human or animal body.
Because the amphiphile has both polar and apolar groups which cluster to form polar and apolar regions, it can effectively solubilise both polar and apolar compounds.
In addition, many of the structures formed by amphiphiles/structuring agents in polar and/or apolar solvents have a very considerable area of polar/apolar boundary at which other amphiphilic compounds can be adsorbed and stabilised.
The formation of non-lamellar regions in the amphiphile/water, amphiphile/oil and amphiphile/oil/water phase diagrams is a well known phenomenon. Such phases include liquid crystalline phases such as the cubic P, cubic D, cubic G and hexagonal phases, which are fluid at the molecular level but show significant long-range order, and the L3 “sponge” phase which comprises a multiply interconnected three-dimensional bi-continuous network of bilayer sheets which lack the long-range order of the liquid crystalline phases. Depending upon their curvature, these phases may be described as normal (mean curvature towards the apolar region) or reversed (mean curvature towards the polar region). Where the spontaneous curvature of the lipid system is close to zero, the structures are typically lamellar, such as uni- or multi-lamellar vesicles/liposomes and where the spontaneous curvature is more negative or positive, cubic, hexagonal and micellar phases typically dominate.
The non-lamellar liquid crystalline and L3 phases are thermodynamically stable systems. That is to say, they are not simply a meta-stable state that will separate and/or reform into layers, lamellar phases or the like, but are the thermodynamically stable form of the mixture.
Both lamellar and non-lamellar systems have been investigated for their properties as carriers and/or excipients for dietary, cosmetic, nutritional, diagnostic and pharmaceutical agents but the non-lamellar systems are thought to have considerable advantages in terms of their high internal surface area between polar and apolar regions. This has led to considerable investigation of non-lamellar phases particularly in controlled-release formulations and for solubilising compounds of relatively low solubility.
As discussed above, a bulk non-lamellar phase is typically a thermodynamically stable system. In addition, this bulk phase may be dispersed in a polar or non-polar solvent to form particles of a non-lamellar (especially liquid crystalline) phase in a bulk solvent. This allows the advantages of bulk non-lamellar phases to be applied in situations where use of a bulk non-miscible phase would cause problems, such as in parenteral applications. Further control of a compound's transport properties and release profile may also be achieved by such a dispersion of non-lamellar particles.
Liquid crystalline or L3 phase can be in or near thermodynamic equilibrium with the excess solvent and may be dispersed into colloidally stable dispersions of non-lamellar particles. Such particles may be fully (i.e. thermodynamically) stable, or may gradually degrade, thereby providing control over the release profile for active agents formulated therewith. The formation of dispersions can be spontaneous or as the result of mechanical force such as shearing or ultrasound. These non-lamellar particles are of considerable interest in the delivery of active agents and have been proposed as carriers for many such actives.
A method for the formation of dispersed particles of non-lamellar phase in solvents such as water is described in U.S. Pat. No. 5,531,925. Such particles have a non-lamellar liquid crystalline or L3 interior phase and a lamellar or L3 surface phase and may also contain active ingredients.
Known particles of liquid crystalline or L3 interior phase may be formed by methods such as adding to this phase a solution of surface-phase forming agent, stirring to form a coarse dispersion and fragmenting the resulting mixture.
In order to assess the presence of a liquid crystalline phase, the prospective liquid crystalline material may be examined by use of small-angle X-ray diffraction (SAX), cryo-Transmission Electron Microscopy (cryo-TEM) or Nuclear Magnetic Resonance (NMR) spectroscopy studies. The sizes and size distributions of the dispersed particles may be examined by light scattering, particularly by use of laser light scattering instruments.
Dispersions containing active ingredients and particularly those for intravenous administration to the human or animal body are desirably colloidal, that is they should be of a particle size no greater than 10 μm, especially no greater than 5 μm and particularly no greater than 1 μMm. If particles within the dispersion exceed this size then the dispersion may not be colloidally stable and may settle or float from the dispersion. There is also a considerable risk of causing embolism when the preparation is administered intravenously. Furthermore, it is desirable that the distribution of particle sizes be narrow to maximise control over the release of any active agent. Where a particulate composition is to be administered by a method other than intravenously (e.g. orally, intramuscularly, subcutaneously, rectally or by inhalation), then the particles need not necessarily be colloidal but it remains advantageous to provide a well characterised and reproducible particle size distribution in order to control the rate of in vivo transport and decomposition of the particles and/or release of the active agents.
The particle size of a particulate composition should also be stable to storage over a considerable period of time. If the distribution of particle sizes changes significantly then the effective transport rate for composition (e.g. due to diffusion and rate of release of any active agent) may be adversely affected. Of still greater concern is the stability of particle sizes in a colloidal dispersion for intravenous administration. If the particle size distribution of such a dispersion is not stable (e.g. to storage and distribution) then large particles may form over time and be dangerous when administered. Even if not directly dangerous, storage instability can cause significant variability in pharmacokinetics, dynamics and/or efficacy.
In addition to control over particle size, it is desirable to maximise the proportion of particles which are in the desired, non-lamellar, phase in order to maximise the beneficial effects of this in terms of loading capacity, protective encapsulation, controlled release, reproducibility etc. The proportion of lamellar particles such as uni- or multi-lamellar vesicles should therefore be minimised.
Known methods for the formation of dispersed particles of non-lamellar phase are highly effective, but typically produce a relatively broad distribution of particle sizes and a considerable proportion of “contaminant” lamellar vesicular particles. Increasing the proportion of fragmenting and/or stabilising agent (e.g. surfactant, copolymer and/or protein) in the formulation or increasing the energy input of the homogenisation process may be used to narrow the particle size distribution but at the expense of increasing the proportion of lamellar particles. There is therefore a considerable need for methods by which a dispersion of non-lamellar particles may be formed having a narrow, preferably colloidal, particle size distribution and a high proportion of non-lamellar particles.
The present inventors have now unexpectedly established that by heating lamellar and/or non-lamellar particles of appropriate composition to an elevated temperature for a short period before cooling to room temperature, the distribution of particle sizes may be narrowed, the stability of the particle size distribution improved and/or the proportion of non-lamellar particles increased.